1. Technical Field
The present invention relates to an apparatus and a method for recognizing a lane, including a center line or edge lines of a road, such as for assisting the driver of the vehicle.
2. Related Art
Drive assist systems in general use include a lane recognition apparatus that recognizes so-called white lines, or a lane, on a road. Such a drive assist system is provided with an in-vehicle camera that acquires an image around the vehicle. An image picked up by the in-vehicle camera is processed by the lane recognition apparatus and a lane is detected from the image.
The drive assist systems may include lane keeping systems or lane departure warning systems. A lane keeping system assists the steering action of a vehicle equipped with the drive assist system (hereinafter simply referred to as “the vehicle”) on the basis of the information on a detected lane so that the vehicle can appropriately travel within the lane. A lane departure warning system calculates a distance between the vehicle and a lane marker on the left and on the right and gives a warning to the driver as to a high probability of departing from the lane if the vehicle nears a lane marker to an extent of exceeding a predetermined threshold.
As some techniques suggest, the position of a lane can be detected with a good accuracy. For example, according to a technique, parameters, such as the lateral displacement of the vehicle in a lane or the curvature of the road, are calculated using the extended Kalman filter (see Patent Document 1).    Patent Document 1 JP-A-2002-109695
If a lane recognition apparatus is capable of recognizing a lane with a good accuracy up to a long distance, a positional relationship between the vehicle and a preceding vehicle can be appropriately obtained. The appropriate grasp of the positional relationship enables appropriate application of automatic braking to prevent a collision. Further, the enhancement in the accuracy of lane departure prediction enables stable lane keeping.
The technique for directly acquiring parameters from a picked up image as disclosed in Patent Document 1 raises a problem that the accuracy of predicting the curvature of a lane tends to be impaired. This is because an image picked by the in-vehicle camera, a distant area in the image in particular, shows only a minute difference between a lane imaged askew due to the curve of the road, and a lane imaged askew due to the vehicle's oblique traveling with respect to the straight road.
In addition, lane recognition up to a long distance allows the recognition process to be easily affected by disturbances attributed to preceding vehicles, backlight and the like, to thereby easily impair the robustness. It is desired that a lane keeping system performs lane recognition of a distant area. On the other hand, a departure warning system is desired to perform lane recognition of a near-field area with accuracy higher than that of the lane keeping system. Lane recognition targeting up to a long distance tends to impair the robustness. Therefore, it has been difficult to achieve a target performance of a departure warning system, which would have been achieved in near-field lane recognition.